Method of improving steel properties by using controlled cooling rates

ABSTRACT

A steel material exhibiting high yield strength and excellent toughness in which fine ferrite-pearlite structure is obtained without producing bainitic structure or martensite. Such properties are obtained by heating up a steel substantially consisting of less than 0.25% C, 0.6 to 2.0% Mn. 0.01 to 0.10% Sol.Al, and 0.01 to 0.2% Nb or less than 0.20% (Nb+V), to Ac3 temperature or above but below 1000*C and then acceleratecooling said steel at a rate of 0.8* to 2.0*C/sec until the transformation is completed.

United States Patent Kozasu et al.

[4 1 Oct. 21, 1975 METHOD OF IMPROVING STEEL PROPERTIES BY USING CONTROLLED COOLING RATES [75] Inventors: Isao Kozasu; Teruhiko Shimizu,

both of Yokohama, Japan [73] Assignee: Nippon Kokan Kabushiki Kaisha,

' Tokyo, Japan [22] Filed: I Mar. 14, 1973 [21] Appl. No.: 340,942

[30] Foreign Application Priority Data Mar. 15, 1972 Japan 47-25801 [52] US. Cl 148/144; 75/123 L; 75/123 N;

[51] Int. Cl C2ld l/l8 [58] Field of Search 148/144, 122, 121, 31.55,

148/12.1,120,11l,ll2;75/123 L, 123 N [56] References Cited I UNITED STATES PATENTS 1,919,983 7/1933 Morrill 148/112 3,098,776 7/1963 Elarde 148/122 3,151,005 9/1964 Alworth et al.. 148/111 3,335,036 8/1967 Yoshida et al. l48/12.1

3,522,114 7/1970 Knuppel et al. 148/111 3,620,856 11/1971 Hiraoka 148/121 3,632,456 l/1972 Sakakura et al. 148/111 3,657,022 4/1972 Kubotera et al. 148/12.1 3,661,656 5/1972 Jarleborg 148/144 3,671,337 6/1972 Kumai et al. 148/112 OTHER PUBLICATIONS Sachs, G. et al.; Practical Metallurgy, ASM, Cleveland, 1940 pp. 466-467. Young, J., Materials & Processes, New York, 1954, pp. 251, 278-279, 290-291, 293-295.

Primary Examiner-Walter R. Satterfield Attorney, Agent, or Firm-William Anthony Drucker [57] ABSTRACT A steel material exhibiting high yield strength and excellent toughness in which fine ferrite-pearlite struc- .8 C1aims,'3 Drawing Figures U.S. Patent Oct.21, 1975 Sheet1of3 3,914,135

U.S. Patent Oct. 21, 1975 Sheet 2 of3 3,914,135

Cooling Rore (C Sec) (AS R||ed) In general, to improve hot rolled structure, and the I refining'of crystal grain size, there is employed a known normalizing process (in which steel is air-cooled after heating up to austenite region and then kept for a re.- quired time. In such a case, there-is no problem when the thickness of said steelis relatively thin, e.g.less

toughness can be produced without adding any special alloying element. The feature of said steel lies in producing ferrite-pearlite structure whose fraction is at least more than 50% by the special heat-treatment. It is, however, confirmed that the yield strength of said steel is still insufficient while notch toughness is considerably improved. On the other hand, in Showa 45-31058. Nb is added to the steel as a strengthening element and fine ferrite-pearlite structure, whose fraction is at least more than 60% is formed. The feature of the above Showa 45-31058 lies in forced cooling,

whereas such forced cooling should be avoided in than 6mm, since the cooling rate, even if by air-cooling, is still high enough.- However, as said thickness becomes'thicker, e.g. more than 10mm, and especially beyond mm, many difficulties tend to appear. That is, there are limits on improving said properties, and especially toughness, because-said rate of air-cooling is Showa -41 1 1 since undesirable bainite or partial martensite transformation tends to be produced. When such low temperature transformation products as bainite are formed in steel even partially, it is certain that said yield strength is lowered and fracture appearance transition temperature deteriorates. It is, in fact, confirmed that the above tendency appears in a steel based on Showa 45-31058, which is cooled by force. The

chemical composition of materials tested-inthe experilowered as said thickness increases. Therefore, an alloying element, e.g. Ni, is further added to said steel or :'properties are shown in FIG.-'2. -v-- merits is shown in Table I and the changes. of physical TABLE I Chemical Composition (Wt.%) Steel C Si Mn P S Cu Cr Nb V- SoLAl A 0.08 0.40 1.26 0.015 0.014 0.043 .13 0.09 0.41 1.31 0.016 0.014 0.058 0.069 C '1 0.09 0.40 1.29 0.015 0.016 g 0.022 0.036 0.13 0 133 1.28 0.013 0.013 0.20 0.08 0.008 0.023 E 0.14 0.21 1.33 0.006 0.009 0.09 0.31 0.025 0.048 0.008 0.17 0.41 1.36 0.017 0.017 0.031

quired, there-are produced other defects. That is,

firstlyfsaid cooling rate is difficultto control. Secondly,

-unevenness ofc'ooling or a known hard spot appear. In

particular, said hard spot isproducedin a location of said steel where many drop prm'itcooling water to *strike atcarIy coolingstage of'cooling.:Needless to say such phenomena bring about n'on-uniformitybf the mechanical. properties and especially deterioration of "toughness. In "a different manner from the above process, sometimes an air-blast c'ooling-is also'employed on said steel. It is obvious however obvious that a cooling rate capable of influencing on said properties of steel is difficult to obtained and, when it is employed without adding an alloying element, there is naturally a limit in improvement of yield-strength and toughness.

Thus, it is a fact that a steel having high yield strength and excellent toughness is very difficult to obtain at a low cost. Therefore, many attempts have been made to do so. For example, an improvement based on the publication of Japanese Patent Application, Showa 35-41 1 l or Showa 45-31058 is a typical instance. According to Showa 35 -4111, a steel having high notch Steels A and Pin the above Table-1 are steels based on Showa 35-4111, and Steel C based on Showa 45-31058. Referring now to FIG. 2, it will be well understood that said yield strength and 50% fracture appearance transition temperature are worsened as cooling rate increases. It should be noted that this tendency appears on the steel based on Showa 45-31058, which is forced cooledin contrast with Showa 354111. In other words, the producing of the bainitic structure, and/or martensite in the Showa 45-31058 steel seems to be unavoidable. Thus, a feasible manufacturing process for steel having high yield strength and excellent toughness without adding a special alloying element or heat-treating e.g.- the known quench-temper (or normalizing) is not yet in existence.

SUMMARY OF THE INVENTION This invention has been developed to overcome the present situation. The features of this invention lie in 5 subjecting a steel consisting of less than 0.25% C, 0.6

BRIEF DESCRIPTIQN OF TH DRAWINGS Other objects and advantages will be apparent from the following descriptionwith reference to the accompanying drawings in which: I I

FIG. 1 is an.explanatory view of an accelerate coolin step with two-phase flow gas (mist).

FIG. 2-is a graph showing a relation between cooling rate and fracture appearance transition temperature and yield strength, and

FIG. 3 shows variation of the state of hardness through plate thickness depending upon cooling rate.

DESCRIPTION OF THE'PREFERRED EMBODIMENT I Referring now to the above-mentioned Table land FIG. 2, it is evident that the physical properties of steels are caused to deteriorate as cooling rate increases, es-

pecially at more than 2C sec.; regardless of the chemical composition of the steels. Experiments were carried out with the following conditions. i

Thicnkess:

Steels A, B, C and F: 40mm Steels D and E: 14.3mm Heating temperature:

Every steel: 900C Cooling rate: varied (average rate at 850C 450C) Cooling method:

Blowing of mist jet consisting of water and air, or

water srpay.

Test of toughness: 2mm V notch Charpy test According to said Table l and FIG. 2, it is clearly shown that both said yield strength and said fracture appearance transition temperature are remarkably improved at a cooling rate of 0.3C to 2C/sec. in the case of Steel C of 40mm thickness and of 06C to 2.0C/sec. in the case of Steels D and E of 14.3mm thickness (Nb is contained in each of these steels). In the above mentioned cases, it was confirmed that fine ferri te-pearlite structure is fully formed with no bainitic structure. it should be noted that when the cooling rate is beyond 2C/sec. said properties are worsened, and furthermore that those of steels A, B and F containing no Nb are little improved. That is, the improvement of yield stress of steels A and F is very and the yield stress of steel B, to which only V is added is little improved and its fracture appearance transition temperature rapidly changes for the worse at a cooling rate of more than l.5C/sec.

Thus, the reasons that the properties of steel are remarkably improved with accelerate cooling at 03C to 2C/sec as mentioned above lie in the following behavior of Nb. That is, firstly, when said steel is heated up, the coarsening of the austenite grain is prevented by fine dispersion of Nb-carbonitride. Secondly, the ferritepearlite structure formed in steel is more refined than that of the air-cooled case, where the cooling rate is 0.3C/sec. in case of said 40mm thickness or :6C/sec in the case of said 14.3mm,'dueto the facts that the Y-a transformation temperature is lowered to some extent and that the grain growth after ferrite transformation is restrained with the presence of Nb. Nb is a very effective element to produce said fine fer-I rite-pearlite structure under the above-mentioned ac-L celerated cooling rate. On the other hand, in a steel I containing no Nb, there is shown little grain refining effect with said accelerate cooling. In a steel containing only V, said accelerated cooling is of no utility, because.

precipitation behavior of V-carbonitride in the cooling I 7 stage changes widely depending upon its cooling rate. As mentioned above, the reason that said properties of steel are lowered by degrees as said accelerated cooling rate increases and is beyond 2C/sec, is based on the. fact that said as bainite is formed or that dislocation density in ferrite increases even if said bainitic structure does not appear. It is confirmed by many experiments that the above phenomena are pronounced in the steel containing no Nb.

Influence of heating rate on said properties of steel D in Table was investigated. The results are shown in I FIG. 2 as tests D-1 and D-2. That is, in test D-l heating was done by an ordinary furnace and in test D-2, by high frequency induction. In such a case, it is needless to say that the heating rate of the high-frequency furnace is higher than that of the ordinary furnace. In FIG. 2, it will be understood that said properties are mark edly improved with the above-mentioned accelerate cooling. Especially improved effects with the highfrequency heating are noticed in addition to those of accelerated cooling. The reason for this is based on the facts that the coarsening of austenite grain and the coalescence and the coarsening of Nb carbonitride are prevented, and accordingly the structure after transformation is more refined.

Thus, in the above-mentioned experiments, it became clear that said cooling rate has a large effect on I said properties of steel and the most suitable range of said cooling rate is rather narrow. This fact shows that said cooling rate should be closely measured. Therefore, a metal sheathed chromel-Alumel thermo couple I C: less than 0.25% SoLAl: 0.01 to 0.10%

Mn: 0.6 to 2.0% Nb: 0.0l to 0.20%

Nb+V: less than 0.2% if necessary.

one or more selected from group consisting of less than 1.0% Ni and Cu, less than 0.5% Mo and Cr, I less than 0.2% Ti and Zr Each of said steels C, D and E shown in Table I is one of steels based on this invention. The soaking temperature range for the above steel is from more than Ac point to l,000C and the cooling rate is limited within the range of 08C to 2.0C/sec in view of the abovementioned experiments- The reasons that chemical composition of a steel I based on this invention is limited as mentioned above,

are, as follows: C and MN: When the C content is beyond 0.25% and the Mn content is beyond 2.0%, ab-

normal micro structure tends to appear and said toughness changes for the worse as if a cooling rate of less than 2.0C/sec is employed. When said Mn content is less than 0.6%, the required strength cannot be obtained. Nb and Nb+V: Even if Nb or Nb+V of more than 0.2% is added, no additional effect appears. When said content is less than 0.01%, no effect appears and it is meaningless. Sol.Al: When Sol.Al content is less than 0.01%, deoxidizing effect and fixing effect of nitrogen does not appear. More than 0.10% Sol.Al brings about little incremental effect and cleanliness of steel become worse.

Heating temperature is within ordinary normalizing temperature range, i.e. more than Ac point and is limited to less than 1,000C. If said temperature is beyond 1,000C. said austenite grain tends to coarsen, and Nbcarbide tends to dissolve into the matrix, consequently, undesirable bainitic structure tends to be formed during cooling.

Cooling rate from the above-mentioned heating temperature is closely limited within the range of 0.8 "to 2.0C/sec. The lower limit, i.e. 0.8C/s ec corresponds to an air-cooling rate for steel plate of to 12mm thickness and, accordingly, a noticeable effect has not yet been exhibited. When said rate is beyond 2.0C/sec, undesirable bainite begins to form even if martensite does not appear. If once said structure is formed, the discontinuous yield phenomenon disappears and the lowering of the yield stress is brought about. At the same time, the 50% fracture appearance transition temperature is raised. Consequently, efforts to improve said properties of steel will be brought to naught.

Such accelerate cooling as mentioned above may be carried out with water-cooling by the common spray nozzle. It is, however, recommended in this invention that a two-phase gas jet in which liquid is atomized is employed. The features of said cooling system with said two-phase gas jet lie in that said cooling is thereby very uniform and is controllable with accuracy. An example of two-phase gas jet system e.g. mist cooling system is shown in the accompanying drawing, wherein numeral 1 is a cooled steel material, 2 a spray nozzle, 3, a gas reservoir 4, a roller table 5 a feed pipe of gas, and 6 a feeding pipe for cooling water. A typical single nozzle arrangement of the above basic configuration is shown in FIG. l-(a), a double nozzle arrangement in FIG. l-(c), and a three nozzle arrangement in FIG. l-(b). Moreover, said FIG. l-(c) also is an example of a reversing mechanism for said cooled material as an arrow shows. These configurations are selected as occasion demands.

The embodiments based on the above-mentinoned process of this invention are as follows. The chemical composition of embodiments is shown in Table 2 and, the obtained physical properties, 'in Table 3 and FIG. 3 respectively.

TABLE 3 Physical Properties mm C/sec llKg Kg/mm Kg/mm Kg-m "C G aircooling 0.3 31.0 44.6 45.4 29.0 54 G 40 1.2 0.06 32.9 46.3 42.1 29,1 90 1.8 0.35 33.1 47.0 42.7 29.6 aircooling allcooling aircooling 1 Steel 2 Thickness 3 Cooling rate 4 Water: Air

5 Yield Stress 6 Tensile Strength 7 Elongation 8 Impact energy (vE.) at 0C 9 50% fracture appearance transition temperature (vTrs).

According to the above Table 3 and FIG. 2 (wherein are given results of the above-mentioned basic experiments, i.e. Table I steel), it will be well understood that the properties based on this process are far more excellent than those of the comparative steel (including comparative process, i.e. air-cooling). That is, in the case of steel of 40mm thickness, said yield stress with accelerate cooling increases by 4 to 8 Kg/mm in comparison with the comparative process in which the cooling rate is 0.3C/sec, i.e. air-cooling. At the same time, 50% fracture appearance transition temperature (vTrs) is improved by about 30C. Also, in the case of TABLE 2 Chemical Composition (Wt. Steel C Si Mn P S Cu Cr Nb V Sol.Al

G 0.08 0.40 1.26 0.015 0.014 0.043 H 0.09 0.40 1.29 0.0l5 0.016 0.022 0.036 1 0.13 0.33 1.28 0.013 0.013 0.20 .08 0.008 0.023 .1 0.14 0.21 1.33 0.006 0.009 0.09 0131 0.025 0.048 0.008

steel of 14.3 mm thickness, said steel shows a similar tendency. An example of variation of hardness through section is shown FIG. 3. In FIG. 3, the upppr is Steel (G) and the lower is Steel (H). Referring to FIG. 3, it is understood that not only said hardness of steel G (not containing Nb) is influenced sensitively by the increase of said cooling rate, but also said yield stress is little improved. On the other hand, in the case of Steel (H) (containing Nb), said hardness is little changed in comparison with that of air-cooled, i.e. normalized, steel. It should be noted that this fact shows that said stable ferrite-pearite structure is fully produced. In FIG. 3, it is understood that the variation of hardness through at section is within the range of :1 (Vicker Hardness). There is no precedent for such uniformity.

Thus, to obtain a steel having high yield stress and excellent toughness, the cooling rate, i.e. accelerate cooling at 08C to 2.0C/sec. should be closely retained. In this way the distortion of steel can be minimized. Needless to say said steel material includes slab, tube, pipe, bar, section steel or the like.

ii. heating the steel of step (i) from A point to 100- 0C, and iii. cooling said heated steel at a rate of 030 to 20C per second, until its transformation is completed, at

a selected rate which yields a product substantially free from martensite having high yield strength, and excellent toughness. 2. The method of preparing an improved steel material which includes the steps of:

i. preparing a steel consisting essentially of l 0 0.25% C 0.6-2.0% Mn 0.01 0.10% Sol.Al 0.01- 0.20% Nb or less than 0.20% (Nb+V) at least one selected from the group consisting of:

0- 1.0% Ni & Cu

0 0.5% Mo & Cr

0 0.2% Ti and Zr. ii. heating the steel of step (i) from AC3 point to l000C, and iii. cooling said heated steel at a rate of 0.3C to 2.0C per second, until its transformation is completed, at a selected rate which yields a product I substantially free from martensite having high yield strength, and excellent toughness.

3. The method of preparing an improved steel material, as claimed in claim 1, wherein said steel has a thickness of about 40 mm.

4. The method of preparing an imrpoved steel material, as claimed in claim 2, wherein said steel has a thickness of about 40 mm.

5. The method of preparing an improved steel material, as claimed in claim 1, wherein said cooling step for a steel of'about 14.3 mm thickness is carried out at a rate within the range 06 to 20C per second.

6. The method of preparing an improved steel material, as claimed in claim 2, wherein said cooling step for a steel of about 14.3 mm thickness iscarried out at a rate within the range 06 to 20C per second.

7. The method of preparing an improved steel material, as claimed in claim 1, wherein said cooling step for a steel of about 10 to 12 mm thickness is carried out at a rate within the range 0.8 to 2.0C per second.

8. The method of preparing an improved steel material, as claimed in claim 2, wherein said cooling step for a steel of about 10 to 12 mm thickness is carried out at a rate within the range 08 to 20C per second. 

1. THE METHOD OF PREPARING AN IMPROVED STEEL MATERIAL WHICH INCLUDES THE STEPS OF:
 2. The method of preparing an improved steel material which includes the steps of: i. preparing a steel consisting essentially of 0 - 0.25% C 0.6-2.0% Mn 0.01 - 0.10% Sol.Al 0.01- 0.20% Nb or less than 0.20% (Nb+V) at least one selected from the group consisting of: 0- 1.0% Ni & Cu 0 - 0.5% Mo & Cr 0 - 0.2% Ti and Zr. ii. heating the steel of step (i) from Ac3 point to 1000*C, and iii. cooling said heated steel at a rate of 0.3*C to 2.0*C per second, until its transformation is completed, at a selected rate which yields a product substantially free from martensite having high yield strength, and excellent toughness.
 2. PREPARING A STEEL CONSISTING ESSENTIAALLY OF 0-0.25%C 0.6-2.0% MN 0.01-0.10% SOL .AL 0.01-0.20% NB OR LESS THAN 0.20% (NB+V) II. HEATING THE STEEL OF STEPS (I) FROM AC3 POINT TO 1000*C, AND III. COOLING SAID HEATED STEEL AT A RATE OF 0.30* TO 2.0*C PER SECOND, UNTIL ITS TRANSFORMATION IS COMPLETED, AT A SELECTED RATE WHICH YIELDS A PRODUCT SUBSTANTIALLY FREE FROM MARTENSITE HAVING HIGH YIELD STRENGTH, AND EXCELLENT TOUGHNESS.
 3. The method of preparing an improved steel material, as claimed in claim 1, wherein said steel has a thickness of about 40 mm.
 4. The method of preparing an imrpoved steel material, as claimed in claim 2, wherein said steel has a thickness of about 40 mm.
 5. The method of preparing an improved steel material, as claimed in claim 1, wherein said cooling step for a steel of about 14.3 mm thickness is carried out at a rate within the range 0.6* to 2.0*C per second.
 6. The method of preparing an improved steel material, as claimed in claim 2, wherein said cooling step for a steel of about 14.3 mm thickness is carried out at a rate within the range 0.6* to 2.0*C per second.
 7. The method of preparing an improved steel material, as claimed in claim 1, wherein said cooling step for a steel of about 10 to 12 mm thickness is carried out at a rate within the range 0.8* to 2.0*C per second.
 8. The method of preparing an improved steel material, as claimed in claim 2, wherein said cooling step for a steel of about 10 to 12 mm thickness is carried out at a rate within the range 0.8* to 2.0*C per second. 